a) Field of the Invention
The present invention relates to an observation optical system for endoscopes which is to be used for observing objects located in liquid environments and has favorably corrected aberrations, especially distortion.
b) Description of the Prior Art
An observation optical system for non-flexible endoscopes, for example, consists of an objective lens system for forming an image of an object, a plurality of relay lenses for consecutively relaying the image and so on.
Known as one of the conventional objective lens systems is the objective lens system disclosed by Japanese Patent Kokai Publication No. Sho 59-226,315. This objective lens system is of the retrofocus type as shown in FIG. 1 in which a lens unit G.sub.1 having a negative refractive power is disposed on the object side of a pupil S and a lens unit G.sub.2 having a positive refractive power is disposed on the image side of the pupil S.
When this conventional objective lens system is combined with relay lenses R.sub.1, R.sub.2 and R.sub.3 as shown in FIG. 2, an image I.sub.1 formed by the objective lens system is relayed as images I.sub.2, I.sub.3 and I.sub.4 by the relay lenses R.sub.1, R.sub.2 and R.sub.3 respectively, and the pupil which determines an aperture of the optical system for endoscopes is relayed simultaneously. An eyepiece lens E is disposed after the image I.sub.4 for observing an enlarged image of the image I.sub.4. The pupil is located at the position indicated by the reference symbol S in the objective lens system, and at the positions indicated by the reference symbols S.sub.1, S.sub.2 and S.sub.3 in the relay lenses. The pupils S.sub.1, S.sub.2 and S.sub.3 generally have a diameter which is equal to an outside diameter of the relay lenses. Accordingly, the optical system for endoscopes has an aperture which is determined almost by the outside diameter of the relay lenses and it is unnecessary to dispose an aperture stop having a light shielding effect at the location of the pupil S.
Further, it is conventionally demanded to configure objective lens systems for endoscopes as the telecentric type which has an exit pupil located at nearly infinite distance. The telecentric type is required for preventing a transmission efficiencies of offaxial light bundles from being lowered in image guides and relay lenses in cases of fiber scopes and non-flexible endoscopes or avoiding problems of color shading, etc. in cases of video scopes using solid-state image pickup devices which are capable of picking up colored images.
The telecentric objective lens system for endoscopes are lens systems which satisfy relationship of f=sin .theta. and produce remarkable negative distortion as the objective lens systems have larger field angles.
Distortion is dependent on an angle of incidence .theta..sub.p of the principal ray on an entrance pupil and an image height is a function of the angle of incidence .theta..sub.p. When distortion is represented by D(.theta..sub.p) and image height is designated by I(.theta..sub.p), distortion D(.theta..sub.p) is defined by the following formula (i): EQU D(.theta..sub.p)=100{I(.theta..sub.p)/f.multidot.tan .theta..sub.p -1}(%)(i)
wherein the reference symbol f represents a focal length of the objective optical system.
I(.theta..sub.p) can ordinarily be expressed in the following form, when A(.theta..sub.p) is a function of .theta..sub.p. EQU I(.theta..sub.p)=f A(.theta..sub.p)
Hence, the above-mentioned formula (i) is transformed into the following formula (ii): EQU D(.theta..sub.p)=100{A(.theta..sub.p)/tan .theta..sub.p -1}(%)(ii)
As is understood from the foregoing description, the relationship between distortion and the angle of incidence of the principal ray is determined solely by the function A(.theta..sub.p) which determines the relationship between the image height and the angle of incidence of the principal ray, and this function represents distortion characteristics of the optical systems.
The above-mentioned function A(.theta..sub.p) is dependent only on imaging relation of a pupil. When the pupil is free from aberrations, i.e., when an objective lens system is assumed to satisfy the sine condition of pupil at all image heights and produces no spherical aberration at an entrance pupil or an exit pupil thereof, A(.theta..sub.p) is determined uniquely by using only a paraxial pupil magnification of the objective optical system as a whole as a parameter and is given by the following formula (iii): EQU A(.theta..sub.p)=sin .theta..sub.p {1-(sin.sup.2 .theta..sub.p)/.beta..sub.p }.sup.-1/2 (iii)
wherein the reference symbol .beta..sub.p represents the paraxial pupil magnification.
In order to maintain the telecentric characteristic, an objective lens system for endoscopes must have a sufficiently high paraxial pupil magnification in absolute. When the objective lens system for endoscopes has a sufficiently high paraxial pupil magnification in absolute, the above-mentioned formula (iii) is approximated by the following formula (iv): EQU A(.theta..sub.p).apprxeq.sin .theta..sub.p (iv)
Hence, distortion D(.theta..sub.p) is expressed by the following formula (v): EQU D(.theta..sub.p).apprxeq.100 (cos .theta..sub.p -1) (v)
As is clear from the formula (v), negative distortion is aggravated as .theta..sub.p is enlarged.
A variety of inventions have hithereto been made to correct the negative distortion in the telecentric objective lens systems for endoscopes. For example, Japanese Patent Kokai Publication No. Hei 2-277,015, No. Hei 3-39,915 and No. Hei 3-200,911 disclosed objective lens systems for endoscopes having corrected negative distortion. Each of these objective lens systems is of the retrofocus type and corrects distortion by using an aspherical lens element in a front lens unit or a rear lens unit thereof.
Furthermore, a lens system for silver salt cameras which has a composition like that shown in FIG. 3 generally satisfies the relationship expressed by the following formula (vi): EQU A(.theta..sub.p).apprxeq.tan .theta..sub.p (vi)
When the formula (vi) is used in the formula (ii), distortion D(.theta..sub.p) becomes zero and it will be understood that a lens system which satisfies the above-mentioned formula (vi) does not produce distortion.
Moreover, known as a lens system used in laser beam printers is an f.multidot..theta. lens system which has a composition shown in FIG. 4 and satisfies relationship of h=f.multidot..theta.. In case of this lens system, the function A(.theta..sub.p) can be expressed by the following formula (vii): EQU A(.theta..sub.p)=.theta..sub.p (vii)
A scanning optical system for laser beam printers ordinarily uses a polygon mirror rotating at a constant angular velocity and an f.multidot..theta. lens which consists of an aperture stop disposed on a polygon mirror. The f.multidot..theta. lens for laser beam printers generally has a large F number and requires nearly no consideration for correction of spherical aberration or coma. In addition, the f.multidot..theta. lens requires no consideration for correction of chromatic aberration since it is used in combination with a light source emitting a monochromatic light bundle. For this reason, optical performance required for the f.multidot..theta. lens can mostly be obtained by selecting a relatively simple composition consisting of a single concave lens element and a single convex lens element as shown in FIG. 4.
In case of a lens system which is to be used for cameras and satisfies the formula (vi), an amount of light to form an image is reduced as .theta..sub.p has a larger value. This relationship is generally referred to as the cosine law. Consequently, the lens system satisfying the formula (vi) is improper for use as an optical system for endoscopes. Also for another reason that the lens component disposed on the object side has an outside diameter larger than those of the other lens components, this lens system is improper for use as a lens system for endoscopes having an outside diameter on which strict restriction is imposed.
In case of the conventional optical system which satisfies A(.theta..sub.p)=sin .theta..sub.p, in contrast, the optical system produces remarkable negative distortion cancelling the above-mentioned cosine law and forms an image which is uniformly bright over the entire range from the central portion to the marginal portion thereof even when .theta..sub.p has a large value.
Accordingly, most of the optical systems for endoscopes which satisfy the sine condition has an excellent characteristic that image brightness is uniform over the entire range from the central portion to the marginal portion. However, such optical systems for endoscopes produce remarkable distortion, form images having marginal portions contracted as compared with central portions, and do not permit accurate measurements or analyses of shapes when used for inspections or observations of objects in the industrial field or constitute causes of erroneous diagnoses in the medical field.
All of the conventional optical systems are designed on a premise that the optical systems are to be used in air or in combination with an object side medium having a refractive index of N.sub.o =1. The above-mentioned objective lens system for endoscopes, cameras and laser beam printers are also supposed for use in air.
However, medical non-flexible endoscopes, especially cystoscopes and throscopes adopted in the field of urinary organs, arthroscopes adopted in the field of orthopedics and specula adopted in the field of examination of parturient women are generally used in practice for observation while flushing water (physiological sodium chloride solution or nonelectrolytic aqueous solution, etc.) to locations to be observed. That is to say, the optical systems of these non-flexible medical endoscopes are used in combination with an object side medium having a refractive index of N.sub.o '.apprxeq.1.333.
In the field of orthopedics, in particular, endoscopes, especially arthroscopes, are widely used for minimally invasive surgery of the articulation of knees and distortion remaining in optical systems of the endoscopes used in water apparently deforms shapes of meniscus and travelling conditions of lattice-like blood vessel systems. An optical system for endoscopes which has corrected distortion permits easily observing that an edge of a knife to be used for the surgical operation is directed upward as shown in FIG. 5A. When the edge of the knife is observed through an optical system for endoscopes in which negative distortion remains, however, it is difficult to judge a direction of the edge of the knife as seen from FIG. 5B, thereby constituting a hindrance to the surgical operation.
A lens system which has a nearly planar surface on the object side, like the objective lens system for endoscopes, produces distortion which is largely different between a case where the lens system is used in air for observation and another case where the lens system is used in water for observation. In a case of an objective lens system which has a planar surface on the object side, for example, a ray is refracted by an object side surface r.sub.1 as shown in FIG. 6A when the objective lens system is used in air (N.sub.o =1), whereas the ray is refracted by the object side surface as shown in FIG. 6C when the objective lens system is used in water (N.sub.o =1.333).
In case of an objective lens system which has distortion completely corrected when used in air, the lattice-like object is observed as illustrated in FIG. 6B. A(.theta..sub.p) of this objective lens system is given by the formula (vi). When an angle of incidence of an optional ray on the first surface r.sub.1 is represented by .theta..sub.o, an angle of emergence on the first surface r.sub.1 is designated by .theta..sub.1 and a refractive index on the side of emergence of the first surface r.sub.1 is denoted by N.sub.1 in FIG. 6A, Snell's law gives the following formula (viii): EQU sin .theta..sub.o =N.sub.1 sin .theta..sub.1 (viii)
When the objective lens system which refracts the ray in water as shown in FIG. 6C is used, we obtain the following formula (ix): EQU 1.333 sin .theta..sub.o '=N.sub.1 sin .theta..sub.1 (ix)
wherein the reference symbol .theta..sub.o ' represents an angle of incidence on the first surface r.sub.1 of the objective lens system when water is an object side medium and the numeral 1.33 indicates the refractive index of water.
From the formula (viii) and the formula (ix) which are mentioned above, .theta..sub.o is expressed by the following formula (x): EQU .theta..sub.o =sin.sup.-1 (1.333 sin .theta..sub.o ') (x)
Further, since a focal length of the objective lens system used in water can be expressed as 1.333 f, the formula (i) is transformed as follows: EQU D(.theta..sub.o ')=100{I(.eta..sub.o ')/(1.333 f tan .theta..sub.o ')-1}(%)
Hence, distortion to be produced by the objective lens system is expressed by the following formula (xi): EQU D(.theta..sub.o =100 [tan {sin.sup.-1 (1.333 sin .theta..sub.o ')/1.333 tan .theta..sub.o }-1] (xi)
FIG. 7 shows a graph illustrating distortion which is produced in a condition of 0.degree..ltoreq..theta..sub.o .ltoreq.70.degree., i.e., when the objective lens system is used in water in a condition of 0.degree..ltoreq..theta..sub.o '23 44.8.degree., FIG. 8A shows a view illustrating an appearance of an image of a lattice-like object in a condition of 0.degree..ltoreq..theta..sub.o .ltoreq.60.degree. i.e., 0.degree..ltoreq..theta..sub.o '.ltoreq.40.5.degree., and FIG. 8B shows a diagram illustrating an appearance of the lattice-like object in a condition of 0.degree..ltoreq..theta..sub.o .ltoreq.70.degree., i.e., in a condition of 0.degree..ltoreq..theta..sub.o '.ltoreq.44.8.degree.. As is seen from these drawings, positive distortion is produced in a larger amount as the objective lens system has a larger field angle. Since the objective lens system which produces distortion as described above forces observers to observe deformed appearances of images, the objective lens system gives very strange impression to the observers who are accustomed to observation through the conventional endoscopes producing the negative distortion.
In the objective lens system for non-flexible endoscopes disclosed by above-mentioned Japanese Patent Kokai Publication No. She 59-226,315, curvature of field is corrected by largely refracting rays with an image side lens component which is used for composing a rear converging lens unit, whereby eccentricity or inclination of the lens component relative to an optical axis produces remarkable variations of aberrations or remarkable adverse influences on images.
In the recent days where endoscopes are used for surgical operations and inserted directly into human bodies in some cases, it is necessary to sterilize the endoscopes sufficiently. As a typical sterilizing method for appliances for surgical operations, it is known to keep the appliances in steam at a high temperature and at a high pressure. If portions of these appliances which are to be brought into direct contact with steam during the sterilization are made of the ordinary optical glass materials, the optical glass materials will be corrected by the steam, thereby rapidly making the appliances unusable. It is known to use, for the portions which are to be brought into direct contact with the steam, cover glass plates made of an optical material free from such corrosion, concretely an artificial sapphire (crystal of Al.sub.2 O.sub.3). When an objective lens system which uses such a cover glass plate has an enlarged field angle, rays will be eclipsed by the cover glass plate which is made of the sapphire.
In addition, there is conventionally known an optical system for non-flexible endoscopes having a composition as illustrated in FIG. 9. This optical system for non-flexible endoscopes is composed of an objective lens system O, and relay lenses R.sub.1, R.sub.2 and R.sub.3, configured so as to form an image of an object within a field lens F by the objective lens system O, and has favorably corrected aberrations. The optical system for non-flexible endoscopes of this type is characterized in that an exit pupil of the objective lens system O is relayed by the field lens F to the relay lenses R.sub.1, R.sub.2 and R.sub.3, and positive curvature of field produced by the relay lenses is cancelled with negative curvature of field produced by the retrofocus type objective lens system O so that curvature of field is corrected within the optical system for non-flexible endoscopes as a whole, and that coma is corrected by disposing a cemented lens component having a cemented surface R which has a concave function in a positive lens unit arranged in the retrofocus type objective lens system O.
The optical system for non-flexible endoscopes illustrated in FIG. 9 has defects that the optical system has a field angle which is variable due to a problem inherent in manufacturing processes and that curvature of field is undercorrected due to an increased number of relaying operations. Speaking concretely, the optical system configured so as to relay a pupil of the objective lens system O by the field lens F has a field angle which is variable due to variations in a spacing between the objective lens system O and the field lens F serving for relaying the pupil, variations in a spacing between the field lens F and the relay lens R.sub.1, variations in thickness of the field lens F and other causes.
Further, the undercorrection of curvature of field in the optical system is caused since positive curvature of field in the relay lens system is proportional to a number of relaying operations. For this reason, curvature of field which is favorably cancelled with negative distortion produced by the objective lens system O when the relaying operations are repeated three times will be undercorrected when the relaying operations are repeated five times.
In the objective lens system shown in FIG. 1 disclosed by the above-mentioned Japanese Patent Kokai Publication No. Sho 59-226,315, the exit pupil of the objective lens system O is located at infinite distance and pupils of the relay lenses are transmitted by disposing a meniscus lens component L.sub.1 having a concave surface on the image side so that the principal ray converged or diverged before the meniscus lens component L.sub.1 is made nearly paralle. The so-called telecentric optical system which is configured so as to relay a pupil to infinite distance by the objective lens system o and the relay lenses R.sub.1, . . . makes it possible to obtain an optical system for non-flexible endoscopes having a field angle which is not varied regardless of the variations in the spacing between the objective lens system O and the relay lens R.sub.1, or almost free from variations. Furthermore, when the objective lens system comprises a meniscus lens component which makes a Petzval's sum negative, the objective lens system has a Petzval's sum which has a negative value larger than that of the optical system shown in FIG. 9, whereby the objective lens system produces curvature of field having a large negative value. In this case, curvature of field can be corrected favorably in the optical system for non-flexible endoscopes as a whole.
In an attempt to obtain a lens system which has a wide field angle, for example, of 120.degree. or larger, the conventional objective lens system for non-flexible endoscopes comprises, at the object side location, a negative lens component which has an image side surface having a too short radius of curvature and can hardly be manufactured in practice. Moreover, it can be conceived to configure an objective lens system which comprises an additional negative lens component as shown in FIG. 10 or is composed of two negative lens components L.sub.1 and L.sub.2. In this case, however, heights of rays become higher on the object side surface of a cover glass plate C as the objective lens system has a wider field angle, whereby the ray indicated by the dashed line in FIG. 10 is eclipsed by the cover glass plate C and a visual field for observation is undesirably eclipsed. When the steam-proof cover glass plate C is omitted for preventing the eclipse of the ray, the lens system will undesirably be corroded during sterilization.